1. Field of the Invention
The present invention relates to a charging system and a charging device for charging a secondary battery and a battery pack provided with a secondary battery.
2. Description of the Background Art
FIG. 6 is a chart showing an operation of charging a secondary battery by a constant-current, constant-voltage (CCCV) charging according to a background part. FIG. 6 shows a closed circuit voltage CCV of the secondary battery, an open circuit voltage OCV of the secondary battery, a charging current Ic, a state of charge SOC of the secondary battery and an internal resistance Ri of the secondary battery in the case of charging the secondary battery, e.g. a lithium ion battery. FIG. 7 is a conceptual diagram showing an equivalent circuit of a secondary battery 100.
The equivalent circuit of the secondary battery 100 shown in FIG. 7 is represented by a series circuit of a voltage source E and the internal resistance Ri. Then, the closed circuit voltage CCV is equivalent to voltages at the opposite ends of the series circuit of the voltage source E and the internal resistance Ri, and the open circuit voltage OCV is equivalent to voltages at the opposite ends of the voltage source E. The secondary battery 100 may be assembled cells in which, for example, a plurality of unit cells of a lithium ion battery are connected in series parallel.
In CCCV charging, charging by constant current I2 is first carried out. In constant current discharge of one hour, the current level that can discharge electricity assumes a nominal capacity NC “1C”. Then, the electric current which multiplied number P of the cells arranged in parallel by 70% of the electric current of the “1C” is I2. In this way, the constant current I2 is charged.
When the closed circuit voltage CCV reaches a final voltage Vf (×the number of the cells arranged in series), a transition is made to a constant voltage (CV) charging area and the charging current Ic is decreased so as not to exceed the final voltage Vf (×the number of the cells arranged in series). When the charging current Ic is decreased to a current value I3 set according to temperature, full charging is judged and the supply of the charging current is stopped. The above charging control method can be read, for example, from Japanese Unexamined Patent Publication No. H06-78471.
In such CCCV charging, the maximum value of the charging current Ic is the current I2 flowing into the secondary battery 100 during the constant current (CC) charging from the start to the end of the charging.
The internal resistance Ri of the battery is a sum of reaction resistance, which results from movements of electric charges caused by the chemical reaction in the battery, and electronic resistance, which is resistance of electrolyte and electrodes. In the secondary battery 100, e.g. the lithium ion battery, if a state of charge SOC (hereinafter, merely “SOC”) is low, an active material on the outer surfaces of the electrodes contracts to increase electronic resistance. If the SOC is high, the active material on the outer surfaces of the electrodes expands to reduce electronic resistance. Thus, the internal resistance Ri has a property of becoming larger as the SOC decreases while becoming smaller as charging proceeds to increase the SOC.
Then, at an initial stage of the charging where the constant current (CC) charging was started, the current I2 flows through the internal resistance Ri when the active material contracts to maximize the internal resistance Ri. Thus, a voltage drop by the internal resistance Ri increases. Since the closed circuit voltage CCV is a sum of the open circuit voltage OCV and the voltage drop by the internal resistance Ri, a difference between the open circuit voltage OCV and the closed circuit voltage CCV is largest at the initial stage of the charging and becomes gradually smaller as the charging proceeds to decrease the internal resistance Ri. It is possible to think of the internal resistance Ri while dividing it into an internal resistance Rim caused at the negative electrode side of the secondary battery 100 and an internal resistance Rip caused at the positive electrode side.
FIG. 8 is a chart showing the open circuit voltage OCV, the closed circuit voltage CCV and a relationship of potentials at the positive and negative electrodes with respect to a lithium reference. In FIG. 8, a voltage V1 represents a voltage drop caused by the flow of the charging current Ic through the internal resistance Rim at the negative electrode side, wherein V1=Rim×Ic. Further, a voltage V2 represents a voltage drop caused by the flow of the charging current Ic through the internal resistance Rip at the positive electrode side, wherein V2=Rip×Ic.
Here, if the internal resistance Ri is zero, i.e. the internal resistance Rim is zero at the start of the charging, i.e. in a state where the SOC is substantially zero, the closed circuit voltage CCV is equal to the open circuit voltage OCV and the negative electrode potential with respect to the lithium reference takes a positive value larger than 0 V. However, the internal resistance Ri is actually not zero and, accordingly, CCV=OCV+V1+V2=OCV+Rim×Ic+Rip×Ic. Then, the negative electrode potential of the secondary battery 100 is decreased by V1=Rim×Ic. Here, since Rim increases because the SOC is lowest at the start of the charging and the charging current Ic is the maximum current I2 from the start to the end of the charging by the constant current (CC) charging, there is a likelihood that V1 is also maximized from the start to the end of the charging and the negative electrode voltage of the secondary battery 100 falls to or below 0 V. If the negative electrode voltage falls to or below V, there has been a problem that lithium precipitates at the negative electrode to deteriorate the secondary battery 100.